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Probability

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by gmat7202011 » Mon Mar 14, 2011 8:21 pm
A carton has 25 bulbs, 8 of which are defective.

What is the probability that if a sample of 4 bulbs are chosen, exactly 2 of them will be defective.

How should i approach this ?
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by 6983manish » Mon Mar 14, 2011 8:36 pm
gmat7202011 wrote:A carton has 25 bulbs, 8 of which are defective.

What is the probability that if a sample of 4 bulbs are chosen, exactly 2 of them will be defective.

How should i approach this ?
Answer ::

We have 25 bulbs , and we need to select just 4 of them , we can do it 25C4 ways.
We have 8 defective bulbs , and we have to select just 2 of them , we can do it 8C2 ways.

Hence the desired probability would be 8C2 / 25C4.
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by srcc25anu » Mon Mar 14, 2011 8:51 pm
should it not be 8c2 * 17c2 / 25c4
because we have to select a total of four, out of which 2 are defective, 2 are fine. so we select 2 defective out of 8 in 8c2 ways and select 2 good ones out of remaining 17 in 17c2 ways and total ways of selecting 4 out of 25 = 25c4
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by 6983manish » Mon Mar 14, 2011 8:56 pm
srcc25anu wrote:should it not be 8c2 * 17c2 / 25c4
because we have to select a total of four, out of which 2 are defective, 2 are fine. so we select 2 defective out of 8 in 8c2 ways and select 2 good ones out of remaining 17 in 17c2 ways and total ways of selecting 4 out of 25 = 25c4
Yes , srcc25anu .

I missed that one, 8c2 * 17c2 / 25c4 should be the right answer.

Thanks
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by gmat7202011 » Mon Mar 14, 2011 9:02 pm
Good one srcc25anu , Thanks
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